Field of the Invention
The invention concerns polycrystalline diamond compact (“PDC”) drill bits used in drilling well bores.
Description of the Prior Art
PDC drill bits are commonly used to drill wellbores in the hydrocarbon industry. These bits can provide efficient drilling speeds and effective cutting. Further, it is possible to recondition worn bits through maintenance such as replacing worn cutting elements.
However, the design of these bits leads to a phenomenon known as “balling.” In general, a PDC drill bit comprises a plurality of stationary (relative to the bit body) blades, into which the cutting elements are mounted. These blades extend outwardly from the bit body, and extend below the bit body before curving generally inward toward the longitudinal axis of the bit.
“Below the bit body” in this context, will be understood to mean “further downhole from the surface than the bit body,” because utilization in directional drilling may place the tool in an effectively horizontal position.
Between the blades are “junk slots” that allow mud and other debris to pass uphole through the drilling fluid flow past the bit. However, the common design of PDC bits creates a cavity, or cone, between the curved-in blades of the tool and the body of the tool. Because the linear speed of the tool decreases as the distance from the longitudinal axis of the tool decreases, mud and other debris that accumulate in the cone are not as readily thrown into the outward flow through the junk slots.
Accordingly, mud and other debris can accumulate in the cone. When such an accumulation occurs, it tends to stick to more debris, creating a “balling” effect. As the ball grows outward, it can impede the cutters on the bit, reducing, and potentially eliminating, drill efficiency.
Attempts to prevent balling have included such efforts as U.S. Pat. No. 7,694,755 to Marvel, et al. which suggests varying the depth of at least one junk slot from that of the other junk slots, presumably in an attempt to provide a flow path to debris closer to the longitudinal axis of the tool. However, such variations must be carefully balanced to prevent rotational imbalance of the tool, and may slow, but not necessarily prevent, the balling phenomenon.
Other attempts to prevent downhole balling in drill bits include the use of emulsifiers or lubricants, as with U.S. Pat. No. 5,586,608 to Clark, et al., and U.S. Pat. Nos. 5,007,489 and 5,099,930, both to Enright et al.
However, despite prior art attempts, the problem of drill bit balling remains. Accordingly, it is desirable to provide a PDC drill bit that can prevent or greatly reduce the effect of balling.